The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
In recent years, a variety of battery technologies have been developed for portable and stationary applications. These include lead acid, lithium ion, nickel/metal hydride, sodium sulfur, and flow batteries, among others. Unfortunately, not one of these technologies provides a sufficiently low cost, long enough cycle life, high enough rate capability, or high enough energy efficiency to provide useful energy storage and power for short-term, transient stationary storage applications. This includes the mitigation of costly transients on the electric grid, short term back up power and load management, regulatory services, and the deferral of investments in other power grid infrastructure components. Batteries may also support off-grid stationary electronic systems, whether by providing short-term backup power, or for facilities not connected to larger grids. Another large, unfulfilled market for batteries is the microhybrid, or stop-start automotive battery. The lead acid batteries currently used for this application must be oversized by an order of magnitude to avoid destruction due to repeated deep discharge cycling, resulting in heavy, expensive systems.
Conventional battery electrode materials cannot survive for enough deep discharge cycles to be used for these transient applications related to the electric grid or microhybrid vehicles. Their rate capability is also limited by poor kinetics for ion transfer and diffusion, or by the formation of new material phases. The use of Prussian Blue analogues, which are transition metal cyanides of the general formula AxPy[R(CN)6]z.nH2O (A=alkali cation, P and R=transition metal cations, 0≦x≦2, 0≦y≦4, 0≦z≦1, 0≦n) as battery electrode materials has been demonstrated. Such electrodes offer longer cycle life, faster kinetics, and higher energy efficiency than any other family of insertion electrodes when operated in aqueous alkali salt electrolytes. However, Prussian Blue analogues have trace solubility in these aqueous electrolytes. The gradual dissolution of Prussian Blue analogue electrodes results in capacity loss of the electrode, limiting the calendar life of the battery. In one case, however, a Prussian Blue analogue was observed to have zero capacity loss after over 5,000 cycles and two months of operation, indicating that under some conditions, these electrodes can be made stable.
What is needed is a system and method for slowing and/or preventing dissolution of electrodes into an operating electrolyte to extend a calendar life of the electrodes.